This invention relates to a new process for embedding rare earth fluorides into silica glasses. The new embedded glasses are useful as optical amplifiers for telecommunications.
All optical amplifiers, and particularly erbium doped fiber amplifiers have experienced explosive deployment in fiber optic telecommunication systems because of the well-recognized advantages that these types of devices have over repeater type amplification schemes. For example, the erbium doped fiber amplifier (EDFA) conveniently operates in the preferred 1550 nm third telecommunications spectral window, has high polarization-insensitive gain, low cross talk between signals at different wavelengths, good saturation output power, and a noise figure close to the fundamental quantum limit. The excellent noise characteristics potentially allow hundreds of amplifiers to be incorporated along the length of a fiber telecommunications link which could then span thousands of kilometers. Optical amplifiers, and particularly EDFAs in contrast to electronic repeaters, are also transparent to data rate, signal format and wavelength over a limited range, making them especially useful for wavelength multiplexed communication systems that simultaneously transmit a large number of signals using different wavelength bands for each signal.
Currently, germania-doped silica and heavy metal fluoride (such as ZBLAN) glasses are used as hosts for rare earth ions, such as Erbium and praseodymium, to make fiber amplifiers. Silica base glasses are chemically and mechanically stable. They easily are fabricated and fused into the germania-doped silica communication fibers. However, silica glasses are inefficient for infrared upconversion because of their large phonon energy. On the other hand, fluoride glasses have low phonon energy, but are very difficult to fiberize. They are also hard to fuse with the silica fibers.
Transparent oxyfluoride glass-ceramics which are comprised of fluoride microcrystals in the mainly oxide glass matrices offer unique properties of high chemical durability of oxide glasses and low phonon energy environment for the rare earth ions of fluoride glasses or crystals. These glass-ceramics are made by melting oxides and fluorides of the cation components to form oxyfluoride glasses. Heat treatment then precipitates out fluoride micro-crystals (ceramming). Because these transparent oxyfluoride glass-ceramics generally have lower melting temperatures, higher refractive indexes and higher thermal expansion coefficients than silica glasses, they may pose a challenge to making fiber amplifiers with pure silica overcladding.
Accordingly, there continues to be a need for CTE-matched optical amplifiers and methods for making such products. In particular, there is a need for new ways for embedding rare earth fluorides into glasses suitable for making optical amplifiers.
Briefly, the present invention relates to a silica-based glass containing rare earth fluoride crystals. In another aspect, the invention relates to a method for embedding rare earth fluorides into silica-based (or germania-doped silica) glasses by solution chemistry. By silica-based I mean glass having silica and/or germania-doped silica as the predominant component.
The rare earth fluoride doped silica preforms of this invention comprise:
wherein X and Y are integers. The sums such as SiO2+GeO2 are fully interchangeable. Each component could range from 0-99% as long as the total SiO2+GeO2 is between 85 and 99%. The sums of Al2O3 and Ga2O3 also are interchangeable. R is alkali or alkaline-earth ions such as Na, K, Li, Ca and Mg.